Technical Field
The present disclosure relates to a sensor of volatile substances with an integrated heater and to a process for manufacturing a sensor of volatile substances.
Description of the Related Art
Known to the art are humidity sensors of a capacitive type, which exploit as sensitive materials particular hygroscopic dielectric materials having an electrical permittivity that varies as a function of the degree of relative humidity. In practice, a sensitive layer of hygroscopic dielectric material is set between conductive structures coupled for forming the electrodes of a capacitor. The capacitance of the capacitor notoriously depends upon the electrical permittivity of the material that is present between the electrodes. Since this varies according to the humidity absorbed by the sensitive layer, the reading of the capacitance of the capacitor supplies a measurement of the level of relative humidity in the environment.
Capacitive humidity sensors are much appreciated for their high sensitivity, good linearity over a wide range of values of relative humidity, low consumption, ease of miniaturization, and low manufacturing costs.
Some known types of capacitive humidity sensors use capacitors with plane and parallel plates. In this case, the electrodes of the capacitor are defined by parallel plates, and the sensitive layer is contained in a volume comprised between the electrodes. One of the electrodes of the capacitor, the external one, has through openings for enabling the environmental humidity to impregnate the sensitive layer. A limit of sensors of this type is represented by the response time, which is rather slow. In fact, the exposed surface of the sensitive layer is small and is limited to the areas corresponding to the openings of the external electrode, which, on the other hand, cannot be increased beyond a certain limit without affecting the overall capacitance of the sensor.
Also sensors based upon comb-shaped and comb-fingered capacitive structures have been proposed. In this case, two comb-shaped, comb-fingered, and coplanar electrodes are formed on a planar dielectric substrate and then coated with a sensitive layer of hygroscopic dielectric material, the electrical permittivity of which varies as a function of the humidity absorbed.
Capacitive humidity sensors may present undesirable phenomena of hysteresis. In fact, when the relative humidity decreases, the hygroscopic dielectric material takes a rather long time, longer than the time taken for absorption, to release the water molecules absorbed.
For this reason, humidity sensors may incorporate heaters thermally coupled to the layer of hygroscopic dielectric material. The heaters enable faster release of the humidity absorbed and thus improvement of the response of the sensors.
A heater may comprise conductive strips, for example of doped polysilicon, and is in general provided in a region underlying the detection electrodes. For instance, the heater may be provided on a substrate and incorporated into an insulating layer on which the detection electrodes are set, which are in turn coated with the layer of hygroscopic dielectric material.
To prevent any electrical contact between the heater and the detection electrodes, the insulating layer should have a certain thickness. This thickness, however, tends to reduce also the thermal coupling between the heater and the hygroscopic dielectric material and thus represents a limit for the performance of the humidity sensor.